Operation of an aerobic bioreactor is highly sensitive to the composition of wastewater treated and the time within which treatment is to be completed. Stated differently, given enough time and no reasonable constraint on the expenditure of energy, the challenge to operate a waste treatment system would be de minimis. Since the challenge is to provide a system which economically satisfies the time-energy sensitivity of its operation, the emphasis in the art is to provide the most efficient means to maintain the activity of a biomass by transferring as much oxygen into the bioreactor's reaction mass (aqueous suspension of biomass, namely, of organic solids and microorganisms) as can be used by microorganisms to biodegrade biochemically oxidizable material. To be commercially acceptable, this must be done within a reasonable period of time, using a small amount of energy, to produce treated water of acceptably high quality. The high difficulty of evaluating such a system with a laboratory bench scale apparatus, led to the testing of the system in a pilot plant on the grounds of the NDH Sandusky automobile plant of General Motors Corp. Wastewater from a Mansfield General Motors plant was also trucked over to the pilot plant for testing because there was a substantial variation in the composition of the wastewater from the two plants.
More specifically, this invention relates to treating a mixture of natural hydrocarbon and synthetic metal-working fluids, fats, oils and greases ("FOG") mixed with synthetic organic and organometallic compounds, some of which are insoluble solids, others emulsifiable liquids, and still others soluble solids and liquids. This mixture in wastewater is referred to herein as "waste fluids". Waste fluids comprise particularly difficult-to-treat components and relatively easy-to-treat components.
The problem is to provide a process which will biologically degrade waste fluids in a wastewater stream which varies from one hour to the next, economically. The stream varies both in mass flow per hour, and in the concentration of waste fluids therein. The problem to be solved is to use a membrane separator which typically operates with constant mass flow per hour, in conjunction with a bioreactor, to provide an effluent which will meet governmental compliance requirements. Further, the ratios of the components of the waste fluids varies during operation. Yet, a successful process requires that it operate essentially continuously substantially without human attention. Still further, the waste production from the process is to be minimized.
The foregoing problems are solved by a process which requires that free oil, and, all solids except finely divided solids, be removed before the wastewater is treated further. It was discovered that the composition of solids in this system are such that those in the size range greater than about 106 .mu.m (140 mesh U.S. Standard Sieves) adversely affect operation of the membranes free oil impairs the selectivity of the membranes by fouling them (wetting out), preventing the passage of water. Solids substantially larger than 106 .mu.m were mainly recalcitrant solids, much of which were inorganic, e.g. metal and carbide particles; and among larger organic particles in the waste fluids, many were of a composition so poorly biodegradable that removing them in a pretreatment proved to be a determinative factor for operating our process successfully. Except for those very small solids which pass through the 140 mesh sieve, the only solids in our bioreactor system are the biomass of microorganisms both dead and alive, and the solids those microorganisms generate.
Another determinative factor was specifying the system so as to generate permeate at a rate greater than that at which solids-depleted feed is introduced to the reactor. This high rate of permeate production makes it possible to provide a permeate recycle, in addition to the recycle of biomass in concentrate. Despite the apparent contradiction of recycling the permeate after going to the trouble of generating it, the permeate recycle is essential for the reactor to operate at constant volume, and to feed a membranous ultrafiltration zone at a constant rate of flow. The reason for the permeate recycle is explained in detail herebelow.
The process incorporates the foregoing features and relies upon the unique operation, in combination, of an ambient pressure aerobic reactor, an equalization tank, and a membrane device tailored to let pass through it, a predetermined amount and size of molecules which are the product of biodegradation.
Wastewater delivered ("delivered wastewater") is pretreated to remove floatable free oil and settleable solids prior to transfer into an equalization tank. One example of a pretreatment device is a corrugated plate interceptor (CPI) although any other devices suitable for removing floatable oils and settleable solids may be used. Alternatively, the removal of floatable free oils and settleable solids may be performed in the equalization tank directly, with suitable equipment. The feed to the bioreactor is taken from the equalization tank and contains FOG having chemical and physical properties quite unlike wastewater containing waste generated by human activities, typical of municipal wastewater. In particular, wastewater from metal-working operations contain any or all of the following constituents: petroleum-based (oil-based) FOG; non-petroleum based (synthetic or semi-synthetic oils) FOG; and organometallic compounds. These constituents vary in biodegradability across the full spectrum of difficulty. It is this feed which was treated in a membrane bioreactor system which was extensively tested at the NDH General Motors automobile plant, in the pilot plant tests which were reported in a paper presented on Oct. 10, 1990 at the WPCF Conference in Washington, D.C.
The basic technology, using a bioreactor with a membrane separator, was disclosed a quarter of a century ago in U.S. Pat. No. 3,472,765 to Budd et al, the disclosure of which is incorporated by reference thereto as if fully set forth herein. They used a well-aerated bioreactor in combination with a microfiltration or an ultrafiltration membrane, not only to avoid the time penalty of gravity settling technology but also to provide essentially solid-free water of high quality ("permeate") to be recovered and the remaining undegraded high molecular weight materials and solids-containing stream ("concentrate") recycled to the bioreactor.
The essential process characteristic of the '765 process was that it maintained a constant reactor volume by varying the feed flow. Further, the organic solids were comminuted but not removed, so they remained in the recycled concentrate. In contrast, the process of our invention maintains a constant reactor volume by maintaining the flow rate of feed to the reactor essentially constant, and, recycling both concentrate and permeate. In our process, essentially no solids greater than about 106 .mu.m enter the reactor, and the only solids in the system are the aforementioned finely divided organic solids, the biomass itself, and the products that biomass generates.
The '765 system was commercialized with limited success in the 70's mainly with respect to human and animal waste, such success arising in applications which were not cost-sensitive. The operation of such a system with delivered wastewater containing waste fluid with FOG from a metal-working plant was unsuccessful because the solids retention time (SRT) and the hydraulic retention time (HRT) were not long enough to degrade the waste fluid.
Though each mechanical component in the system is known, the combination used in our process is found to be effective if operated as described hereunder to treat delivered wastewater which contains a very high concentration of FOG. The bioreactor is operated to maintain a predetermined concentration of FOG and total suspended solids ("TSS"), and the membrane device is operated as an ultrafiltration membrane at low pressure, in the range from about 170 to 1035 kPa (25 psig to 150 psig). Such operation results in a controlled high mass flow of solids-containing concentrate as a recycle stream.
The mass flow from the bioreactor is surprisingly high, yet (i) provides a long solids retention time (SRT), and enough liquid as is required per unit of air entrained, to degrade the FOG in delivered wastewater, and also (ii) completes degradation of the organic waste with a hydraulic retention time of less than 5 days, preferably less than 48 hr, in the bioreaction system. The key to providing the foregoing is to retain emulsified pollutants for a period longer than the liquid residence time or hydraulic retention time (HRT) of the reactor, based on the wastewater flow rate.